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Plant factory and modern greenhouse

Special Issue:
Plant factory and modern greenhouse

Editor: Eiji Goto and Yasuomi Ibaraki
April 2021

The Japanese Society for Horticultural Science, which publishes Hort. J., has organized this special issue to further increase the attractiveness of this journal.

The theme of the third special issue is “Plant factory and modern greenhouse”, which includes many aspects, from crop physiology under controlled environments, monitoring of plant physiological status, and application of light-emitting diodes to advanced cultivation technology.

Plant factories, sometimes called vertical farms, are a closed plant production system with artificial light that was proposed, developed, and implemented in Japan during the 1980s. During the 1990s and since then, the products from these factories have received high evaluation by the food service industry, to which they primarily cater. Since the late 2000s, plant factory technology has been introduced worldwide. In 2020, plant factories were commercially operated in many countries.

Modern greenhouses are an advanced system equipped with environmental control and hydroponics and supporting year-round production. High-quality vegetables, flowers, ornamental plants, and fruit trees are produced in greenhouses. Mechanization and automation have recently been installed in greenhouses. Modern greenhouses are regarded as advanced production systems in the field of horticulture.

To achieve ideal plant production in these systems, it is important to determine and comprehend plant growth characteristics and responses that are unique to controlled environments and then apply the acquired knowledge to actual cultivation. From these perspectives, related research has become more important for the next generation of horticulture. Therefore, we have planned this special issue of Hort. J.



In contrast to fluorescent lamps and high-power sodium lamps, the use of light-emitting diode (LED) lamps enables the control of not only photosynthetic photon flux density (PPFD) at the plant level, but also the relative spectral photon flux density distribution (RSPD) of light because of the variety, even at different times of day, of producible light emitted by LEDs of different types. Effects of the spectral photon flux density on plant growth and morphology have been investigated using several types of LEDs and plant species. However, few studies on lighting methods with time-varying PPFD or RSPD have been published to date. In this paper, we summarize the effects of time-varying PPFD on the net photosynthetic rate (Pn) and those of time-varying RSPD on plant growth and morphology. Detailed modeling studies have been conducted on the reactions of the photosynthetic pathway under time-varying PPFD at a cycle of milliseconds to seconds. The results of these modeling studies and actual measurements of Pn under pulsed light clearly indicate that pulsed light is not advantageous to improve Pn. Although the integrated PPFD of blue and red light was unchanged, the growth of leaf lettuce was promoted by asynchronous irradiation with blue light and red light compared with growth under simultaneous irradiation. We think that blue-light monochromatic irradiation promotes leaf elongation through leaf expansion as a primary factor in the enhancement of plant growth. In addition, changes in leaf photosynthetic capacity caused by blue-light monochromatic irradiation may be involved in plant growth promotion. An increasing number of studies have investigated the effects of time-varying RSPD on plants. However, the mechanisms underlying these effects remain to be elucidated.

Hongjia Xu, Masahumi Johkan, Satoru Tsukagoshi, Toru Maruo

Pages: 90 (2): 154–160. 2021.|doi: 10.2503/hortj.UTD-207


Recently, the number of patients with chronic kidney disease has increased rapidly and kidneys with loss of the K-defecating function have been observed. Thus, providing vegetables with low potassium is an urgent unmet need. In this study, two cultivation methods were used to cultivate lettuce (Lactuca sativa L.) with low K concentrations. One method, dubbed LKEC, was based on electrical conductance management and the K supply was stopped at the end of cultivation. The other method, dubbed LKQM, was based on nutrient quantitative management, and the nutrients required for low-K lettuce were quantitatively supplied. Meanwhile, control lettuce with a normal K concentration, known as CK, were cultivated with electrical conductance management. Compared with CK, both low K treatments reduced the yield by nearly 20% without any visual deficiency symptoms. There was no significant difference between LKEC and LKQM in terms of plant growth. LKQM-treated lettuce contained lower Na and required less fertilizer than LKEC lettuce. Moreover, these plants adapted to K deficiency stress by absorbing more cations to maintain osmotic pressure. N declined with decreasing K. This suggested that the quantitative management method in low-potassium lettuce production reduced the potassium content in the lettuce plants to the same level as the EC management method, and significantly reduced the sodium content compared to EC management.

Sakura Takahashi, Jingai Che, Naomi Horiuchi, Hnin Yin Cho, Siaw Onwona-Agyeman, Katsuhiro Kojima, Masaaki Yamada, Isao Ogiwara

Pages: 90 (2): 161–171. 2021.|doi: 10.2503/hortj.UTD-238


Low-potassium crops are required for patients with kidney disease, and research on the production of low-potassium vegetables using hydroponics has been conducted. However, there are few studies on low-potassium fruit trees because the soil is generally cultivated. This study focused on blueberries that can be cultivated in a pot, by examining the production of low-potassium blueberry fruits cultured with fertigation in a greenhouse. In a pot culture using peat moss medium, the potassium levels were restricted from the flowering stage and from the fruit coloring stage, causing a decrease in the potassium content of the fruits by 53 and 35%, respectively, when compared with the control. A urethane sponge-based medium with free nutrient leaching was then evaluated to determine whether the potassium content of fruits decreased with short-term potassium restriction. The results showed a reduction in potassium content of 48% when potassium was restricted in the fruit coloring period. In addition, potassium was restricted for five months to determine whether long-term potassium restriction could further reduce the potassium content of fruits. The fruit potassium content did not differ between the second and fifth months after the potassium restriction, although symptoms of potassium deficiency appeared in mature leaves. From these results, it was suggested that the pot culture with fertigation was effective in producing low-potassium blueberry fruits, and the fruit potassium content can be halved by short-term potassium restriction using the urethane sponge-based medium. However, long-term potassium restriction was not effective in producing low-potassium blueberry fruits due to the appearance of symptoms of potassium deficiency.

Yoshiyuki Muramatsu, Satoshi Kubota

Pages: 90 (3): 239–246. 2021.|doi: 10.2503/hortj.UTD-R016


Here, we describe the current status of energy-saving techniques in greenhouse heating and countermeasures for high summer temperatures, and then introduce an outline and the performance of a newly developed New Root-zone Environmental Control System (N.RECS) and the effects of this system on flower production. Temperature control techniques used in protected horticulture include those that control the air temperature of the entire greenhouse space and those that control the local temperature of the plants. The running costs of the air temperature control techniques are expensive and the cooling capacity in the summer are inadequate for actual cultivation. Local temperature control techniques have relatively low running costs and can adequately cool plants during high temperature periods, but they are crop-specific and not versatile. Therefore, we developed a N.RECS that can heat and cool the root-zone of various plants with low running costs. N.RECS was produced by combining aluminum heat exchange panels, insulated pot trays and an air-source heat pump cold/hot water supply system. By using N.RECS, the root-zone temperature was maintained at 25°C when the air temperature was about 5°C during the winter and cooled to below 23°C when the air temperature was about 35°C during the summer. Root-zone heating of six potted flowers and cut flowers in the winter promoted growth and flowering and achieved energy savings of about 30% compared to conventional heating.

When the two flower species, which are susceptible to high temperature, were grown under high temperature conditions, about half of the plants died, but root-zone cooling allowed all the plants to grow healthily. Therefore, it is concluded that N.RECS is a practical system that allows for both energy-saving cultivation in the winter and cooling cultivation in the summer. In addition, root-zone temperature is an environmental control parameter that should be controlled in future agricultural production; by applying root-zone temperature control as a standard technology for agricultural production, the growth of plants can be controlled more flexibly and more energy can be saved.

Takafumi Watabe, Masaru Homma, Dong-Hyuk Ahn, Tadahisa Higashide

Pages: 90 (3): 247–254. 2021.|doi: 10.2503/hortj.UTD-263


To improve the yield of sweet peppers in Japan, we investigated the relationships between plant growth characteristics and fruit yield, with a particular focus on dry matter production. Fruit fresh weight (i.e., fruit yield) was highly correlated with fruit dry weight. Aboveground dry matter production was correlated with both light use efficiency and intercepted photosynthetically active radiation. Although the number of fruits was correlated with the fruit set ratio, no correlation was found between the number of fruit and fraction of dry matter distributed to fruit. The correlation between the fraction of dry matter partitioned to fruit and fruit dry weight varied by plant stage, potentially due to the yield fluctuation called “flush”. Together, these findings suggest that to improve the yield of sweet peppers in Japan, we should focus first on improving light use efficiency.

Masaru Higashiura, Mitsuru Douzono, Yuichi Uno, Masahito Yamanaka

Pages: 90 (3): 255–262. 2021.|doi: 10.2503/hortj.UTD-258


The effects of End-of-Day (EOD)-cooling on floral differentiation and cut flower characteristics in carnation were verified using both short-term model-based experiments in a growth chamber and long-term demonstrative experiments in a greenhouse. In the model experiment, flower buds were undifferentiated until 43 days after pinching in both control and End-of-Night-cooled plants. In contrast, in the EOD-cooling treatment, every set reached the reproductive phase by 22 days after treatment. At the same node position, a three-stage advancement in floral differentiation was observed after EOD-cooling, but not after the other two treatments. This suggested that EOD-cooling promoted both floral differentiation and development in carnation. For the demonstrative experiment, significant differences between EOD-cooled and control plants varied each year in days to flowering, node order of flowering, and stem weeping angle. Because environmental factors could affect variation, correlations between the diurnal temperature range and the difference in each survey component were examined using regression analyses. Moderate (days to flowering) and a strong (node order of flowering) correlations were found with the diurnal temperature range. Thus, the EOD-cooling treatment was advantageous in suppressing delayed flowering over a large diurnal temperature range during floral differentiation. The difference in stem weeping angle showed a strong correlation with the difference in minimum temperature during the stem elongation stage. Improving the stem quality by EOD-cooling tended to be effective under high minimum-temperature conditions.

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